WO96/01688 relates to a stratified charge internal combustion engine comprising at least one cylinder having at least one intake valve per cylinder, and two manifolds having branches supplying first and second gas streams to at least one intake valve of each cylinder, the two streams entering the cylinder separately through different regions of the valve so as to produce a stratified charge within the engine cylinder, the first manifold supplying a stream comprising air within which fuel to be burnt is dispersed and the second manifold supplying a stream comprising dilution gases, in which the two manifolds are connected separately to respective sources of air and dilution gases, each controlled by separate throttles. The dilution gases could be exhaust gases from an EGR system.
Depending on the relative degree of throttling applied to the two separate throttles, these arrangements may cause different equilibrium Pressures in the plenum chambers of the two manifolds and such pressure difference would set up balancing flows between the branches of the two manifolds at the intake port ends where these branches are connected at the vicinity of the intake valve. The balancing flows are undesirable when a stratified charge is required because they blur the distinction in gas composition of the two streams which should be kept separate from one another until the two streams enter the combustion chamber.
In WO96/10688, it is proposed to use a check valve in each branch of the first manifold to prevent the balancing flows in one direction. GB-A-2 306 194 proposes ganging the throttles and operating them in unison to ensure a fixed ratio between the air and dilution flows through the respective manifolds, the flow ratio being such that the pressure drops along the two sets of manifold branches are substantially equal so that there is no pressure difference between the plenum chambers of the two manifolds.
GB-A-2 306 194 describes an engine in which the relative velocities of the two streams at the intake valve as they enter the combustion chamber are kept constant so as to maintain a fixed pattern of charge motion within the combustion chamber which minimises mixing between the two streams and thereby conserve stratification for as long as possible during the intake and compression strokes of the engine.
In WO97/01397, it is proposed to use a flow obstructing means in each branch of the first manifold to adjust the relative volume flows through the two manifolds in order to maintain the same pressure drop along the branches of the two manifolds while allowing the velocity ratio of the two streams passing over the intake valve to be selected at a desired value according to the resulting volume flow ratio and the local flow cross-sections of the two streams at the intake valve.
All the above co-pending patent applications depend on the volume flow ratio between the air and the dilution streams in the respective manifolds being set at a predetermined value at which there is no balancing flow between the two sets of branches at the intake port ends and at which the velocity ratio of the two streams at the intake valve is maintained at a predetermined value for minimum mixing. This limits the control of the two streams to a very specific narrow range over which both conditions are met and any deviation from that range would reduce the effectiveness of the eventual charge stratification within the combustion chamber.
WO95/22687 represents the closest prior art to the present invention. The embodiment in the latter reference described by reference to its FIG. 5 is a stratified charge internal combustion engine comprising at least one cylinder having at least one intake valve per cylinder, and two manifolds having branches supplying first and second gas streams to at least one intake valve of each cylinder, the two streams entering the cylinder separately through different regions of the valve so as to produce a stratified charge within the engine cylinder. The first stream supplied by the first manifold comprises air within which fuel to be burnt is dispersed and the second stream supplied by the second manifold comprises dilution gases. Each of the two manifolds has a plenum chamber and the plenum chambers of the two manifolds are connected to a common air supply throttle. The plenum chamber of the second manifold is additionally connected to an exhaust gases recirculation (EGR) pipe from the exhaust system of the engine through an EGR throttle, the EGR pipe supplying exhaust gases directly into the second stream to form part of the gases in the second stream.
In the latter reference, the pressures In the two manifold plenums are not matched at all times with the result that cross flow between the manifolds can occur through the intake ports, which if allowed to occur would detract from the charge stratification.